1. Field of the Invention
The present invention is in the field of electronics, and more specifically relates to a circuit for reducing the amount of noise entering an RF receiver from an antenna.
2. The Prior Art
The present invention is a circuit which in normal use would be inserted between an antenna and a radio-frequency (RF) receiver, such as a receiver for frequency modulated (FM) signals. Accordingly, the device must be capable of passing the signal without distorting it, while at the same time removing noise that has been picked up by the antenna. In many cases of interest, this noise takes the form of noise spikes which are superimposed on the desired signal and which generally are much larger than the signal.
Many devices are known in the art that can be used for limiting, or clipping, an applied signal. For example, amplifiers and cathode followers typically produce output that are linearly related to the input for relatively small input signals, but when the input signal becomes larger, the devices are incapable of producing a proportionately large output. Once the maximum level of output has been reached, further increases in the input do not produce an increase in the output, and the output signal is said to be clipped.
The use of an amplifier in the present application is undesirable ultimately because there is no well defined level that is clipped. Instead, typically, the response curve of an amplifier makes a gradual transition from the linear region to the limiting region, and therefore signals less than the limited signals are severely distorted. Such distortion is unacceptable in the present application.
It is also known in the art that diodes can be used to clip a signal. Use is made of the property of a diode to conduct in one direction but not to conduct in the other direction. Compared to an amplifier, the diode has the advantage the signal that is passed is a faithful reproduction of the applied signal.
Known diodes are unsatisfactory for the present application. Diodes which are capable of responding fast enough tend to be excessively noisy, and this defeats the purpose of the invention which is to remove noise. At present, the more suitable diodes are many times more expensive than the components used in the present invention.
Other approaches such as notch filters or the use of blanking pulses are unacceptable because the devices interrupt the signal.
Thus, there exists a need for a relatively simple and inexpensive circuit which can be inserted between the antenna and the input of an RF receiver for the purpose of removing noise from the received signal while passing the signal with a minimum of distortion.